In vitro transcription (IVT) uses bacteriophage DNA-dependent ribonucleic acid (RNA) polymerases (e.g., SP6, T3 and T7) to synthesize template-directed mRNA transcripts. Problems in the IVT reaction can result in complete failure (e.g., no transcript generated) or in transcripts that are the incorrect size (e.g., shorter or longer than expected). Specific problems associated with IVT reactions include, for example, abortive (truncated) transcripts, run-on transcripts, polyA tail variants/3′ heterogeneity, mutated transcripts, and/or double-stranded contaminants produced during the reactions.
RNA polymerases exhibit three phases of transcription—initiation, elongation and termination. During the initiation phase, the RNA polymerase binds to a specific promoter DNA sequence, opens the DNA duplex and feeds the template strand into the active site. T7 RNA polymerase, for example, forms a structure referred to as initiation complex, which includes a six-helix bundle sub-domain (the promoter binding domain) that interacts with the promoter to initiate DNA duplex melting. While bound to the promoter, the polymerase produces many short (truncated) transcripts from 2-12 nucleotides (nt) in length, a process often referred to as abortive synthesis/initiation. The truncated RNA transcripts cannot be converted to full-length transcripts by RNA polymerase and become by-products that accumulate during transcription. After the transition to the elongation phase and release of the promoter, the polymerase proceeds down the DNA template producing a full-length RNA transcript.
During the elongation phase, RNA polymerase often continues to transcribe DNA beyond the point at which termination should be initiated, generating longer than expected RNA transcripts (“run-on transcripts”). T7 RNA polymerase, for example, adds nucleotides to the end of a transcript before ‘falling off’ the template. Studies suggest that more than 70% of transcripts generated by T7 RNA polymerase in vitro may be run-on transcripts. In some cases, these aberrant RNA products are twice the length of the encoded sequence. Because run-on transcription is stochastic, there is often great 3′ heterogeneity among products in a given IVT reaction. This 3′ heterogeneity is problematic for downstream applications, such as ligation reactions, which are dependent on RNA transcripts of a defined length and/or nucleotide composition.